Field of the Invention
The present invention relates to image capture apparatuses and control methods thereof, and particularly relates to techniques for correcting effects of optical characteristics of a lens unit on an image.
The present invention also relates to lens units that make it easy for image capture apparatuses to correct effects that optical characteristics of the lens unit have on an image.
Description of the Related Art
There is constant demand for image capture apparatuses such as digital cameras to output images with high image quality, and the optical characteristics of lens units used to form subject images are one cause of image quality deterioration. Limb darkening, distortion, chromatic aberration of magnification, and so on can be given as examples of optical characteristics that cause image deterioration, but it is difficult to implement a lens unit that completely eliminates such characteristics. Accordingly, techniques that correct image deterioration caused by optical characteristics by applying image processing to captured images (that is, optical correction techniques) are known.
Meanwhile, the optical characteristics vary within a single lens unit as well, depending on the focal length (angle of view of a zoom lens), shooting distance (focusing distance), optical parameter values such as the aperture value, and so on. Thus when using image processing to correct image deterioration caused by optical characteristics, it is necessary to use optical correction values that correspond to the optical parameters set during shooting in order to achieve an accurate correction.
Japanese Patent Laid-Open No. 2005-286482 proposes a method for determining an optical correction amount based on a polynomial approximation generated from the transition in a distortion correction amount that is based on the zoom position (focal length) of a zoom lens.
With a fixed-lens image capture apparatus, optical correction values can be found in advance for all sets of optical parameter values. A configuration that stores only discrete sets that are actually required based on a storage capacity or the like and finds sets that are not stored through interpolation is possible, and such a configuration makes it comparatively easy to ensure correction accuracy.
On the other hand, in the case of an interchangeable-lens image capture apparatus, a large number of types of lens units can be attached, including special lenses such as macro lenses, shift lenses, and so on.
As mentioned above, the optical characteristics of a single interchangeable lens will differ for each set of a plurality of optical parameters. In a typical lens, it is possible to uniquely find optical correction values based on sets of the focal length, shooting distance, and aperture value, but due to an increased variety in lens units, it is sometimes necessary to employ other optical parameters in order to identify the optical correction values. This will be described hereinafter.
FIG. 2 is a diagram illustrating an example of a relationship between a magnification rate and a shooting distance in a single focus macro lens capable of shooting from life-size magnification to n-times magnification. Although the shooting distance is the same value at a magnification of m and a magnification of n in this example, the optical characteristics change as a result of the magnification changing, and thus the optical correction value is different when the magnification is m and when the magnification is n.
FIGS. 3A to 3D are diagrams illustrating examples of limb darkening properties of the macro lens having the characteristics shown in FIG. 2 and results of correcting the limb darkening using optical correction values found based on the shooting distance illustrated in FIG. 2. Specifically, how the brightness (light amount) of an image obtained by shooting a subject having a uniform luminosity changes depending on the image height, which corresponds to a distance from the center of the image, is illustrated, taking the brightness of the center of the image as a value of 100.
FIGS. 3A and 3B illustrate the limb darkening characteristics of the macro lens when the magnification is m and when the magnification is n, respectively. Although both cases exhibit a decrease in the light amount as the image height increases (that is, as the image progresses from the center toward the periphery), the characteristics are different. Accordingly, for at least one of the magnification of m and the magnification of n, the optical correction value for limb darkening that should be used cannot be identified from sets of the focal length, shooting distance, and aperture value.
FIGS. 3C and 3D illustrate examples of results of correction using optical correction values identified without taking the magnification into consideration. In this example, limb darkening is properly corrected in the case where the magnification is m, but the correction is insufficient in the case where the magnification is n, leading to improper correction being carried out. In this manner, depending on the lens unit, there are cases where a proper optical correction value cannot be determined using only the information of the focal length, shooting distance, and aperture value.
To perform proper optical correction on an image shot using, for example, a macro lens having the characteristics illustrated in FIG. 2, information for identifying the magnification is necessary. For example, the magnification can be mechanically set by manually manipulating a zoom ring for changing the magnification provided on the lens unit, and a proper optical correction value can be identified if the magnification that has been set is known.
Accordingly, to apply the proper optical correction to images that can be shot by a variety of types (models) of lens units in an interchangeable-lens image capture apparatus, it is necessary to obtain the optical parameter values required to identify optical correction values for each lens unit model.
However, the conventional technique disclosed in Japanese Patent Laid-Open No. 2005-286482 does not take into consideration cases where the types of optical parameters required to identify the optical correction values differ from lens unit model to lens unit model, and thus does not provide a solution to the aforementioned problem.